When I got a meat thermometer a few years ago, I was startled at how low optimal cooking temperatures are for beef, less that 140 degrees Fahrenheit. Also, as a kid, I remember reading that the desert floor in Arizona gets up to 180 degrees Fahrenheit, which is much higher. I haven't been able to confirm that, and frankly find it a bit doubtful, but if it's even close, it does imply that an an animal carcass lying on the African plains for a couple of days may be legitimately cooked.
Now, I'm not a biologist, and this is pure speculation, but I'm imagining that if proto-humans picked up a diet consisting largely of medium-rare carrion (probably killed by other predators), this might explain why humans prefer cooked meat and why they lost jaw size (because cooked meat is a lot easier to chew).
Has there been any research along these lines?
Addendum: published temperatures in Arizona seldom get above 120 degrees Fahrenheit, but those measurements are taken in the shade at about 5 feet above the ground. What I'm talking about is measurement of the temperature of sun-warmed rocks on the desert floor. I still think 180 is a little high, but 140 wouldn't surprise me at all.